1. Field of the Invention
The present invention relates to an active matrix display device and an electronic appliance using the active matrix display device.
2. Description of the Related Art
In recent years, as a display device such as a liquid crystal panel, an active matrix (active driving) display device in which each pixel is selectively connected to a data line (or a signal line) through a corresponding switching element so that a potential of each pixel electrode is controlled has been used in many cases. As such a switching element, a thin film transistor (TFT) has been widely used. Such an active matrix display device using a TFT has a problem in that hot carriers are generated due to a voltage applied to the TFT and thus characteristics of the TFT are degraded. When the TFT characteristics are degraded and a threshold voltage is changed, the timing of writing data to a pixel may be off or a defect in writing data may occur because the TFT is not turned on. In order to prevent such degradation of TFT characteristics, an LDD structure in which a lightly doped drain region (or an LDD region) is provided between a channel formation region and a drain region and/or a source region or a GOLD (gate overlapped drain) structure is generally adopted so that an electric field applied to a TFT is reduced. However, there is a problem that adopting these structures increases manufacturing steps or causes variation in TFT characteristics.
Patent Document 1 (Japanese Published Patent Application No. 2002-196358) discloses a liquid crystal display device in which an enough voltage to be applied to a capacitor of a liquid crystal in a pixel (that is, a capacitor formed with a pixel electrode, a counter electrode, and a liquid crystal) can be kept while a potential to be applied to a data line is suppressed to be low to reduce power consumption of the liquid crystal display device. The liquid crystal display device has an storage capacitor in addition to the capacitor of the liquid crystal in the pixel. One terminal of the storage capacitor is connected to one terminal of the capacitor of the liquid crystal in the pixel and connected to the data line through a switching element (TFT). The other terminal is connected to a capacitor line in which a potential can be varied. For example, when a switching element is turned on while an High level potential is applied to the data line, charges in accordance with the High level potential are stored in both the capacitor of the liquid crystal in the pixel and the storage capacitor. After that, the switching element is turned off and at the same time, a potential of a capacitor line connected to the other terminal of the storage capacitor is raised, so that charges in accordance with the potential difference between the raised potential and the original potential is distributed to the capacitor of the liquid crystal in the pixel. Accordingly, an effective value of a voltage to be applied to the capacitor of the liquid crystal in the pixel can be greater than that of the potential to be applied to the data line, so that a voltage high enough to drive liquid crystals (i.e., align liquid crystals) can be obtained. That is, in the liquid crystal display device disclosed in Patent Document 1, the potential to be applied to the data line can be lower than a potential for driving the liquid crystals and a voltage applied to a TFT is also low by just that much; therefore, degradation of the TFT can be prevented.
However, in the liquid crystal display device disclosed in Patent Document 1, a potential of the other terminal of the storage capacitor is required to be controlled through the capacitor line. Therefore, there has been a problem that a signal for driving the capacitor line is required to be generated separately and thus the structure becomes complicated.